Electrostatic Discharge (ESD) is the sudden flow of electric current between two objects at different electric potentials. This is usually due to a build up of charges on one of the objects (or opposite charges on the two objects), which is discharged when the objects come into contact or close proximity. When the electrical field between two objects is sufficient to cause dielectric breakdown in the air separating them, a spark occurs. However, ESD can occur less noticeably when two objects simply contact and the potential difference is simply discharged through conduction. Static build up commonly builds up through tribocharging, which is the separation of charges when two contacting objects are separated.
ESD is a major concern in the use and manufacture of electrical and electronic goods. The sudden discharge of a potential difference of even 12 v can destroy some devices such as transistors. In manufacturing, a typical solution is to ground all components including the operator and tools such that any static is discharged to ground through wire straps.
Ion Sensitive Field Effect Transistors (ISFET) (including Chemically sensitive Field Effect Transistors (ChemFET) and Enzyme Field Effect Transistors (EnFET)) are transistors designed to detect a species in a fluid sample. The charge of the species in proximity or contact with the transistor affects the operation of the transistor, which can be monitored using electrical instrumentation. The top layer of the transistor may be coated with a sensing layer to target a particular species.
Such transistors suffer from ESD, particularly those featuring a floating gate structure. As the name implies, a floating gate, is not electrically connected to the remaining parts of the transistor, so as to make the gate sensitive to ion charges. However this also makes the structure vulnerable to ESD strikes which may leave residual charges in the structure. The electrostatic discharge (ESD) event will cause a charge to be trapped in the floating gate, causing a large threshold voltage shift of the ISFET and even non-reversible damage to the device. Once the threshold voltage shifts, it increases the difficulty of instrumentation design to read the proper threshold voltage in the expected range. ESD can also damage ISFETs by physically and chemically degrading materials or leaving residual charges on various structures such as oxides. As a result, a form of ESD protection for ISFETs is very necessary.
WO9520243 (Baxter) discloses a protection circuit for an ISFET to protect the device from an ESD event to the liquid. The circuit is made up of conventional protective elements, integrated onto a silicon chip in a non-CMOS process.
The known ESD protection schemes either fail to provide sufficient protection or require extra manufacturing steps after a standard CMOS process, making the device costly. The present inventors have appreciated this problem and invented a novel device that provides cost effective ESD protection in an unmodified CMOS process.